The present invention relates to an apparatus for protecting the rotor of a turbomachine, such as a steam or gas turbine or the like, from overspeeding. More specifically, the present invention relates to a backup overspeed trip apparatus that relies on the pressure produced by a pump driven by the turbomachine shaft to determine when a predetermined speed has been exceeded.
Steam turbine power plants typically employ electro-hydraulic control systems that perform a variety of functions, including tripping--that is, shutting down on an emergency basis--the turbine when certain conditions arise. Such conditions include those indicating imminent damage to the turbine--for example, low bearing oil pressure, rotor overspeed, and high condenser pressure. Typically, steam turbines are tripped by closing the throttle valve that controls the introduction of high pressure steam to the turbine. Such throttle valves typically employ a hydraulic actuator. However, since it is important to close such valves as quickly as possible upon tripping, the throttle valve is spring loaded to close. Thus, pressure from a hydraulic fluid must be exerted on the valve operator to keep the valve open. This hydraulic pressure is maintained in a closed loop system by a pump driven by the turbine rotor.
Typically, sensors for bearing oil pressure, condenser pressure, etc. are incorporated into a trip control block. These sensors are coupled to a trip valve that is in flow communication with the hydraulic fluid supplied to the throttle valve actuator. A trip is accomplished by actuating the trip valve so as to dump the hydraulic fluid to a vented drain tank, thereby dropping the pressure to the throttle valve actuator so that the spring automatically closes the throttle valve. In addition to the trip control block, separate trip devices are also typically provided by mechanical and electrical overspeed trip devices.
Traditionally, a lockout device was incorporated into the hydraulic system that allowed the trip devices--that is, the trip block and the mechanical overspeed trip--to be temporarily isolated from the throttle valve actuator, thereby allowing the trip devices to be tested without tripping the turbine. However, this left the turbine unprotected should a trip condition arise during testing. Even during the relatively short time period necessary to test the trip control block, is unwise to leave the turbine unprotected from a rotor overspeed. A rotor overspeed, if unchecked, can cause the rotor to fly apart, resulting in substantial damage to the turbine and surrounding equipment. In a nuclear power plant, such a rotor failure can have catastrophic consequences.
Consequently, during the time the lockout device is actuated, protection against a rotor overspeed condition was provided by an electrical tachometer that transmitted a signal to a solenoid operated trip valve. This trip valve dumped hydraulic fluid supplied to the throttle valve actuator to a drain, thereby closing the throttle valve if the speed exceeded a predetermined value. Unfortunately, such backup valves have been known to malfunction during testing of the trip control block, thereby permitting a damaging rotor overspeed condition to occur.
It is therefore desirable to provide an overspeed protection device that can not be disabled by the traditional trip lockout device and that operated independently of the mechanical and electrical tripping devices heretofore used.